1. Field of the Invention
The present invention relates generally to electronic circuits and, more specifically, the present invention relates to sampling signals with electronic circuits.
2. Background Information
In the electronic circuit industry, there is a continuing effort to increase circuit speeds as well as decrease circuit device sizes. As circuit designs become more advanced, circuits become faster, smaller and are able to operate with less voltage and power. In switched-capacitor circuitry, there is technique known as double-sampling, which enables faster analog-to-digital conversion.
To illustrate, FIG. 1 is circuit 101 illustrating double-sampling using switched-capacitor circuitry. As shown, circuit 101 includes an operational amplifier (op-amp) 103 having a non-inverting input coupled to ground and an inverting input coupled to sample and hold (S/H) circuit 105 and S/H circuit 107. S/H circuits 105 and 107 are a type of switched-capacitor circuit and are coupled to alternatingly sample an input signal vin during opposite phases of a clock signal. As shown, S/H circuit 105 is coupled to the inverting input of op-amp 103 through switch 123. S/H circuit 107 is also coupled to the same inverting input of op-amp 103 through switch 137.
In operation, switch 123 is closed while switch 137 is opened, and switch 137 is closed while switch 123 is opened. During normal operation, switches 123 and 137 are never both closed or opened simultaneously. When input signal vin is being sampled by S/H circuit 105, switches 117, 119 and 121 are closed and switches 123, 125, 127 and 129 are opened so that the input signal vin is sampled onto capacitors 111 and 109. When S/H circuit 105 is evaluated by op-amp 103, switches 117, 119, 121 and 129 are opened and switches 123, 125 and 127 are closed so that the voltage stored on capacitor 111 is evaluated by op-amp 103 through the switched connection provided by switch 123 and capacitor 109 provides a feedback capacitor coupled to the output of op-amp 103.
Similarly, when input signal vin is being sampled by S/H circuit 107, switches 131, 133 and 135 are closed and switches 137, 139, 141 and 143 are opened so that the input signal vin is sampled onto capacitors 113 and 115. When S/H circuit 107 is evaluated by op-amp 103, switches 131, 133, 135 and 143 are opened and switches 137, 139 and 141 are closed so that the voltage stored on capacitor 115 is evaluated by op-amp 103 through the switched connection provided by switch 137 and capacitor 113 provides a feedback capacitor coupled to the output of op-amp 103.
The idea behind double-sampling is that in an analog switched-capacitor system, twice the amount of data can be processed if two sets of analog sampling circuitry are interleaved. Known double-sampling approaches are based upon the ability to switch in and out two identical sampling stages, such as S/H circuits 105 and 107 through switches 123 and 137, respectively. The switching action of switches 123 and 137 reduces the overall performance of circuit 101 and as technology improves and supply voltages are scaled down, double-sampling is virtually unusable in low-voltage or low gain designs.
Known problems with circuit 101 include the fact that the inverting input of op-amp 103 is never discharged between evaluations of S/H circuits 105 and 107. Consequently, residual charge from a previous evaluation is left on the inverting input of op-amp 103 to taint or contaminate a subsequent evaluation. Another problem that limits the performance of circuit 101 is that the capacitive loads of S/H circuits 105 and 107 are both tied to the inverting input of op-amp 103. In particular, it is appreciated that even though switches 123 and 137 are never both closed simultaneously during normal operation, the capacitive loads of S/H circuits 105 and 107 are nevertheless both capacitively coupled to the inverting input of op-amp 103 since switches 123 and 137 are implemented with transistors. As a result, the maximum switching speeds of circuit 101 are reduced due to the relatively large capacitive load coupled to the inverting input of op-amp 103
Methods and apparatuses for a double-sampling a signal are disclosed. In one embodiment, a circuit according to the teachings of the present invention includes an op-amp having four input terminals. Two of the input terminals are tied to ground and the other two terminals are coupled to S/H circuits through unswitched connections. The S/H circuits are coupled to sample an input signal. Since the connections to the S/H circuits are not switched, the problems associated with having to switch in and out the S/H circuits that plague the prior art double-sampling techniques are reduced. Additional features and benefits of the present invention will become apparent from the detailed description and figures set forth below.